Page 315 - Chemical engineering design

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CHEMICAL ENGINEERING
shifts in position in the series can be expected in other electrolytes, but the series for sea
water is a good indication of the combinations of metals to be avoided. If metals which are
widely separated in the galvanic series have to be used together, they should be insulated
from each other, breaking the conducting circuit. Alternatively, if sacrificial loss of the
anodic material can be accepted, the thickness of this material can be increased to allow
for the increased rate of corrosion. The corrosion rate will depend on the relative areas
of the anodic and cathodic metals. A high cathode to anode area should be avoided.
Sacrificial anodes are used to protect underground steel pipes.

7.4.3. Pitting
Pitting is the term given to very localised corrosion that forms pits in the metal surface.
If a material is liable to pitting penetration can occur prematurely and corrosion rate data
are not a reliable guide to the equipment life.
Pitting can be caused by a variety of circumstances; any situation that causes a localised
increase in corrosion rate may result in the formation of a pit. In an aerated medium the
oxygen concentration will be lower at the bottom of a pit, and the bottom will be anodic
to the surrounding metal, causing increased corrosion and deepening of the pit. A good
surface finish will reduce this type of attack. Pitting can also occur if the composition
of the metal is not uniform; for example, the presence of slag inclusions in welds. The
impingement of bubbles can also cause pitting, the effect of cavitation in pumps, which
is an example of erosion-corrosion.

7.4.4. Intergranular corrosion
Intergranular corrosion is the preferential corrosion of material at the grain (crystal) bound-
aries. Though the loss of material will be small, intergranular corrosion can cause the
catastrophic failure of equipment. Intergranular corrosion is a common form of attack
on alloys but occurs rarely with pure metals. The attack is usually caused by a differ-
ential couple being set up between impurities existing at the grain boundary. Impurities
will tend to accumulate at the grain boundaries after heat treatment. The classic example
of intergranular corrosion in chemical plant is the weld decay of unestablished stainless
steel. This is caused by the precipitation of chromium carbides at the grain boundaries in
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a zone adjacent to the weld, where the temperature has been between 500 800 C during
welding. Weld decay can be avoided by annealing after welding, if practical; or by using
low carbon grades (<0.3 per cent C); or grades stabilised by the addition of titanium or
niobium.

7.4.5. Effect of stress

Corrosion rate and the form of attack can be changed if the material is under stress.
Generally, the rate of attack will not change significantly within normal design stress
values. However, for some combinations of metal, corrosive media and temperature, the
phenomenon called stress cracking can occur. This is the general name given to a form